The invention relates generally to methods of purifying a polymer. More particularly the invention relates to methods of purifying a polymer by using a mixture of solvents.
Some methods used for the preparation of polymeric material employ a phase transfer catalyst and/or alkali metal salts of aromatic dihydroxy compounds. The polymeric material is generally subjected to a devolatilization extrusion to obtain a polymer substantially free of the polymerization solvent that is employed during the polymerization process. The presence of residual catalyst during devolatilization extrusion can lead to elevated color in the isolated resin. It is known in the art that residual salt could lead to the formation of haze in polymer pellets.
Typically the process of isolating the polymeric material from a polymerization reaction mixture includes the steps of diluting the polymerization mixture (concentration: about 30 weight percent of polymeric material based on the total weight of polymerization reaction mixture) with a suitable solvent and filtering the resultant diluted mixture. However, in some instances dilution and cooling results in a polymer rich bottom phase and a solvent rich upper phase. Further cooling may cause the polymer rich phase to become more and more viscous until it is almost solid-like. It is not desirable to have this type of phase separation during polymer filtration and aqueous extraction. This phase change is completely reversible if the solution is re-heated and concentrated back to its original state. Heating and concentration results in the substantial dissolution of the polymeric material in the solvent. However filtering or extracting the reaction mixture at a temperature greater than 100° C. and at concentrations of 30 weight percent of polymeric material may not be feasible. Further it may not result in the effective removal of the residual catalyst and the salt.
The present invention provides, among other benefits, a simple and yet elegant solution to this problem.